When in a wireless communication network based on the LTE (Long Term Evolution) radio access technology (RAT) specified by 3GPP (3rd Generation Partnership Project) a UE (user equipment) initially accesses the wireless communication network, the UE needs to acquire what is called system information (SI). This is typically accomplished via broadcast of a certain information in each cell. The broadcasted information includes a primary synchronization signal (PSS) and a secondary synchronization signal (SSS), which may be used by the UE to obtain frequency and time synchronization. The PSS and SSS also encode the physical cell identity (PCI). After the physical layer synchronization and PCI detection, the UE is capable of performing channel estimation using the constantly broadcasted cell specific reference signals (C-RSs) and, consequently, finally decode the SI. The PSS and SSS are respectively transmitted in the first and sixth subframes within a radio frame. Accordingly, the PSS/SSS and C-RSs are always broadcasted by the network. These are used by the UE to synchronize to a given cell and enabling channel estimation.
The SI is broadcasted in each cell by System Information Blocks (SIBs), each of which contains a set of functionally related parameters. The SIB types that have been defined include, a Master Information Block (MIB), which includes a limited number of the most frequently transmitted parameters which are essential for the UE's initial access to the network, a System Information Block Type 1 (SIB1), which contains parameters needed to determine if a cell is suitable for cell selection, as well as information about time-domain scheduling of the other SIBs, a System Information Block Type 2 (SIB2), which includes common and shared channel information, System Information Blocks of Type 3 to 8 (SIB3-SIB8), which include parameters used to control intra-frequency, inter-frequency and inter-RAT cell reselection, System Information Block Type 9, which is used to signal the name of a Home eNodeB (HeNB), System Information Blocks of Type 3 to 8 (SIB3-SIB8), which includes Earthquake and Tsunami Warning Service (ETWS) notifications and Commercial Mobile Alert System (CMAS) warning messages, System Information Block Type 13 (SIB13), which includes MBMS (Multimedia Broadcast Multicast Service) related control information, System Information Block Type 14 (SIB14), which is used to configure Extended Access Barring (EAB), System Information Block Type 15 (SIB15), which is used to convey MBMS mobility related information, and System Information Block Type 16 (SIB16), which is used to convey GPS (Global Positioning System) related information. This list of SIB types has been expanding over the years, and this expansion may be expected to continue as the 3GPP LTE RAT evolves.
Some of the SI is defined as being “essential information”, e.g., the information contained in the MIB, SIB1, and SIB2. For UEs which are EAB capable, the information in SIB14 is also considered as “essential information”. Here, “essential information” is considered to be information that the UE should acquire before accessing the wireless communication network.
In the LTE RAT, the SI, i.e., the MIB and the SIBs, is constantly broadcasted, but depending on the type of information, different periodicities are used. For example, the MIB and SIB1 may be broadcasted with periodicities of 40 ms and 80 ms. Furthermore, for the MIB the transmission is repeated four times during each broadcast period, i.e., every 10 ms. The SIB1 is also repeated four times within each broadcast period, i.e. every 20 ms, but with a different redundancy version for each transmission. For other SIB types, the time-domain scheduling may be dynamically adapted. In particular, each SIB may be is transmitted in a periodically-occurring time-domain window, while physical layer control signaling indicates in which subframes within this window the SI is actually transmitted. The scheduling windows of the different SIBs, referred to as SI-windows, are consecutive, i.e., without overlaps or gaps between them, and have a common length that is configurable. The SI-windows can include subframes in which it is not possible to transmit SIBs, such as subframes used for the SIB1, and subframes used for the uplink in TDD (Time Division Duplex Mode).
As can be seen, the way of transmitting the SI in the LTE RAT may result in a significant amount of signals which are constantly broadcasted.
To increase efficiency of fifth generation (5G) wireless communication networks, a concept was suggested which is based on a layered transmission of access information (see, e.g., “A Clean Slate Radio Network Designed for Maximum Energy Performance” by P. Frenger et al., presented on the IEEE 25th Annual International Symposium on Personal, Indoor, and Mobile Radio Communications (PIMRC), Washington, D.C., Sep. 2-5, 2014). In this concept, user equipments (UEs) are provided with access information by using broadcasted access information tables (AITs) and broadcasted system signature sequences (SSSs), wherein each SSS may be used to identify information from the broadcasted AIT. The AIT may for example define settings concerning a how a UE shall access the system, e.g., by a random access procedure, concerning how the UE can be reached by the system in a paging procedure, or concerning more advanced settings, such as related to beam forming or link adaptation. The AITs are typically transmitted with long periodicity, while the SSSs are typically transmitted more frequently. Typically each access node, e.g., a base station, will transmit an SSS which allows the UE to identify the information applicable to this access node from the AIT. The AITs do not need to be transmitted by every access node. For example, a base stations serving a macro cell may transmit both an AIT and an SSS, while a base station serving a small cell within a coverage region of the macro cell may transmit only an SSS. Accordingly, the AIT will typically include entries defining various configurations which apply to various access nodes. The AIT may therefore have considerable size, so that in view of resource efficiency it is generally desirable to broadcast the AIT at a relatively low update rate.
However, when broadcasting the AIT at a low update rate, situations may occur where the AIT last received by the UE is no longer valid, e.g., because a certain configuration as identified by the SSS received by the UE has been changed by the network, but the updated AIT was not yet broadcasted to the UE or because the UE has moved to another area where another AIT is valid, but this other AIT was not yet received by the UE. In some situations, it is also possible that the last SSS received by the UE does not point to the correct configuration in the AIT, i.e., that the SSS or the mapping of SSSs to entries of the AIT is no longer valid. Before the UE can access the wireless communication network, it may thus need to wait until it has received a valid AIT and a valid SSS.
In order to avoid latency resulting from a need to wait for receiving a valid AIT or SSS, the UE may also use the previously received AIT and/or SSS even though in may be no longer valid. However, this involves a risk of the UE attempting to access the wireless communication network on the basis of an inappropriate configuration, which may cause various problems, such as failure of the access attempt or network-side misconfigurations which impact operation of the wireless communication network also with respect to other UEs. In other situations, using the configuration determined from the outdated AIT or SSS may be possible without a significant risk of such problems.
In the LTE RAT, the SI normally changes only at specific radio frames whose System Frame Number (SFN) is given by SFN mod N=0, where N is configurable and defines the period between two radio frames at which a change may occur, also known as modification period. Before a change of the SI, the UE may be notified by means of a Paging message including a SystemInfoModification flag. Further, a change of the SI may be indicated by a value tag in SIB1 which is incremented every time one or more SI message changes.
If the UE receives a notification of a change of SI, it starts acquiring SI from the start of the next modification period. Until the UE has successfully acquired the updated SI, it continues to use the existing parameters. This may be acceptable if changes of the SI are much less frequent than transmission of the SI. However, if the SI is transmitted less frequently, such as in the above-mentioned case of using the AIT for resource efficient transmission of SI, or if changes of the SI occur more frequently, accessing the wireless communication network based on invalid SI may be problematic from the perspective of the individual UE and also from the network perspective. For example, an excessive number of failed access attempts may adversely affect user experience and cause additional signaling which consumes network resources.
Accordingly, there is a need for techniques which allow for efficiently controlling access to a cellular network on the basis of invalid SI.